I know Waiting has been discussed before, and I have not gone through all old posts on the topic, so I apologize if this has been suggested before.

When a task is moved to Scheduled we get a pop up calendar to select the date (and time) it is scheduled for.

How about doing something similar when moving a task to Waiting by dragging it (it would be redundant when editing the task manually). The pop up could include the Context, Tag and Assign to fields, making it very quick and easy to enter information on who or what the task is waiting for.

This could be done without making any structural changes such as enabling assignment to non-doit-users, and it would continue to work as intended if that feature is ever introduced in the future.

-Tom